As shown in FIG. 1, a conventional AC-DC switching power converter 10 has a pair of AC power input terminals 12 and 14 for connecting to an AC power source, a high-frequency filter capacitor CX connected between the AC power input terminals 12 and 14 for filtering high-frequency signals to prevent high-frequency switching noises from interfering the power network, a bridge rectifier 16 for rectifying the AC input voltage to generate a DC voltage Vdc, and a voltage converter 18 for converting the DC voltage Vdc into a regulated output voltage Vo. When the AC power source is removed, the high-frequency filter capacitor CX will have a residue DC voltage thereon, whose level is equal to the voltage of the AC power source at the moment that the AC power source is removed, and thus may be the maximum voltage of the AC power source, e.g. 264√{square root over (2)}V. Such a high voltage stored in the filter capacitor CX can bring about risks of electric shock. Traditionally, a bleeding resistor RB is connected in parallel to the high-frequency filter capacitor CX, to release the residue voltage to a safe range in a specified time. However, the bleeding resistor RB causes additional power loss. Particularly, if the AC power source is 264 Vac, the power consumed by the bleeding resistor RB is (264V)2/RB, which is expected to be as high as some tens of milliwatts.
In addition, when the AC power source is lower than a normal range and enters brownout state, the AC-DC switching power converter 10 may get damaged. An existing approach is to use a voltage detector with a resistor voltage divider to determine whether the AC power source is in brownout state. However, the resistor voltage divider leads to increased power loss because of its high voltage drop.
Therefore, it is desired a voltage detector for detecting a voltage state without increasing power loss.